FGF21 belongs to the FGF19 subfamily of atypical fibroblast growth factors (FGFs) with metabolic rather than mitogenic effects. FGF21 binds and activates FGF receptors (FGFR1c, FGFR2c and FGFR3c) but only in the presence of the non-signaling co-receptor beta-klotho (BKL). Tissue specific expression of BKL determines the metabolic activity of FGF21. FGF21 transgenic mice are resistant towards diet-induced obesity and have increased longevity. FGF21 is a metabolic regulator of energy expenditure, glucose and lipid metabolism, with a great potential to reverse bodyweight, hyperglycaemia and dyslipidaemia in obese patients with diabetes and dyslipidaemia.
FGF21 suffers from in vivo instability due to proteolysis, and as much as half of the endogenous circulating human FGF21 is inactive. The loss of activity is due to degradation of the C-terminal, the majority of these metabolites terminate at P171 rather than S181. Protection against metabolic breakdown in the C-terminal region is therefore desirable for a therapeutic FGF21 molecule. Engineering of the C-terminal region may protect against degradation, however so far such engineering has come at the cost of lowered or lost potency of the engineered FGF21 compound. The N-terminal region of FGF21 binds to FGFRs while the C-terminal region of FGF21 binds to BKL. Truncations of C-terminal amino acids lead to significant loss of potency.
PEGylation in position 180 of [180C]FGF21 results in dramatic reduction in in vitro activity (J. Xu et al, Bioconjugate Chemistry (2013), 24, 915-925). The Fc fusion protein resulting from attaching Fc to the C-terminus of FGF21 is much less potent than native FGF21 and the N-terminal Fc fusion of FGF21 (Hecht et al, PLoS One 2012, 7(11), e49345).